Background. Prostitution is the most important source of transmission of AIDS and sexually transmitted diseases in Asia. We developed and evaluated the sustainability of an intervention to increase condom use and reduce gonorrhea among brothel-based sex workers in Singapore. The intervention focused on developing sex workers' negotiation skills, educating clients, and mobilizing support from peers and health staff in promoting condom use. Methods. A pretest-posttest design with one intervention site (n = 124) and another comparable control site (n = 122) was maintained for 5 months followed by a time series design to follow up the intervention group for 2 years. Results. At 5 months, the intervention group improved significantly in negotiation skills and were almost twice as likely as controls to always refuse unprotected sex (adjusted rate ratio 1.90, 95% CI I 1.22-2.94). Gonorrhea incidence declined considerably by 77.1% in the intervention group compared with 37.6% in the controls. Consistent refusals of unprotected sex in the intervention group increased from 44.4% at baseline to 65.2% at 5 months, 73.6% at 1 year, and 90.5% at 2 years with a corresponding decline in gonorrhea. Conclusion. Sustained condom use with a corresponding decline in gonorrhea was achieved by a behavioral and environmental intervention for sex workers.